(1) Field of the Invention
The invention is related to a flexbeam unit for a multi-blade rotor of a rotary wing aircraft, said flexbeam unit comprising a plurality of flexbeam elements with the features of claim 1.
(2) Description of Related Art
A flexbeam unit is used in a so-called bearingless or a so-called hinge- and bearingless rotor system to connect rotor blades of a multi-blade rotor of a rotary wing aircraft to a rotor shaft or an associated rotor hub of the aircraft. During operation, i.e. rotation of the multi-blade rotor, the flexbeam unit must withstand and transfer tremendous centrifugal forces that the rotor blades apply thereto, while permitting their flapping, pitch and lead-lag motions. Therefore, the flexbeam unit comprises special, in particular fiber reinforced composite material flexbeam elements that are flexible enough in torsion to allow twisting for blade movement without discrete bearings in the case of a bearingless or a hinge- and bearingless rotor system.
These flexbeam elements usually possess lead-lag-soft regions that permit motions of associated rotor blades in a bearingless or a hinge- and bearingless rotor system in the lead-lag direction. The lead-lag-soft regions thus constitute fictitious vertically oriented axes, so-called virtual lead-lag hinges, about which the rotor blades execute forward and backward lead-lag motions. Furthermore, these flexbeam elements realize flapwise-soft regions that enable flapping of the associated rotor blades in the vertical direction and, thus, constitute fictitious horizontally oriented axes, so-called virtual flapping hinges, about which the associated rotor blades execute upward and downward flapwise motions in a bearingless or a hinge- and bearingless rotor system. The distance between these virtual flapping hinges and the axis of the rotor shaft is referred to as the flapping hinge distance.
Moreover, only in a hinge- and bearingless rotor system, these flexbeam elements usually comprise torsion weak regions which enable low-force torsional motion of the flexbeam elements for inducing pitch angle adjustments of the rotor blades, and which allow limiting an associated length of the flexbeam elements. Such torsion weak regions must be resistant against lead-lag and flap shear forces and provide a required stiffness in lead-lag and flapping direction for dynamic reasons. Furthermore, such torsion weak regions should have a small cross section in order to reduce an associated drag of these regions.
The document DE 199 15 085 A discloses a rotor blade for a bearingless rotor that encompasses at its inner end toward an associated rotor head a flexbeam element that permits flapping or lead-lag motions, as well as an angular deflection about an associated torsion axis. The flexbeam element furthermore transfers the centrifugal forces of the rotor blade to the rotor head. The torsionally flexible region of the flexbeam element is located inside a torsionally stiff control cuff, by way of which control motions are introduced into a lift-generating blade region of the rotor blade. The control cuff is relatively stiff, but the flexbeam element has portions that are flexible so as to form a virtual flapping hinge, lead-lag hinge, and a torsion axis, which respectively enable flapping, lead-lag pivoting, and torsional movements of the rotor blade. The inboard end of the control cuff is secured to a root end of the flexbeam element near the rotor head to prevent lateral displacements there between. Damping elements are preferably arranged laterally next to the flexbeam element in an associated lead-lag plane, and are secured on the one hand to the control cuff, and on the other hand to a securing plate that is connected to the flexbeam element and the rotor blade.
However, the virtual flapping hinge and the torsion axis are arranged in series, i.e. one after another in longitudinal direction of the flexbeam element. Thus, the flexbeam element is unnecessarily prolonged and its aerodynamic performances are decreased.
The document WO 94/27866 A discloses a flexbeam element for a bearingless rotor that comprises an admixture of composite plies embodying specific characteristics, i.e. this flexbeam element is fabricated from composite plies having comparatively good ballistic tolerance characteristics and from composite plies having a high stiffness-to-weight ratio. More specifically, the flexbeam element has a flat and flexible cross section profile over its entire longitudinal extension and comprises continuous unidirectional fiberglass plies having fiber orientations of 0°, unidirectional fiberglass plies of varying lengths having fiber orientations of 0°, and graphite cross plies of varying lengths having fiber orientations of ±45°.
However, the flat and flexible cross section profile of the flexbeam element over its entire longitudinal extension is disadvantageous for embodying a lead-lag hinge, such that lead-lag pivoting of an associated rotor blade could lead to comparatively high tensions in an outer edge of the flexbeam element. This may lead to an undesired distortional buckling and, thus, with respect to underlying rotor dynamics to a disadvantageous coupling between torsion and lead-lag pivoting motions.
The document DE 196 20 427 A discloses a flexbeam element with a dampening device made of elastomer layers and intermediate layers. The dampening device is arranged in slot-shaped notches of a support beam, said slot-shaped notches being required to provide a required elasticity of torsion. This arrangement provides a space-saving housing of a large elastomer volume in the support beam structure itself. Because of such division into a plurality of slots and individual dampers extending in the direction of the main bending plane of the support beam, this arrangement ensures a homogeneous or uniform shear deformation and a large-surface linking of the individual elastomer layers directly to the bending-deformed support beam sections. As a result both a high-level dampening effect and a long useful life of the elastomer damper are achieved.
However, this flexbeam element is complex and difficult to manufacture due to the integration of the dampening device.
The document U.S. Pat. No. 5,690,474 A discloses a composite flexbeam element having a pitch region which includes a core laminate of unidirectional fiberglass material and face laminates of unidirectional graphite material bonded to mating surfaces defined by the core laminate. The core laminate and the face laminates define an aspect ratio which is greater than or equal to ten and define chamfered edge surfaces. Each chamfered edge surface defines a critical acute angle with respect to a given flapwise bending neutral axis of the pitch region and further defines a lateral edge disposed at a given vertical distance from the flapwise bending neutral axis. Furthermore, a combination of unidirectional and off-axis composite materials can be used for realization of the flexbeam element, wherein plies of the composite material are interleaved.
However, this flexbeam element also comprises a flat and flexible cross section profile over its entire longitudinal extension, which is disadvantageous for embodying a lead-lag hinge, such that lead-lag pivoting of an associated rotor blade could lead to comparatively high tensions in an outer edge of the flexbeam element.
The document FR 2 041 747 A discloses a rotor construction comprising a plurality of rotor blades, a rotor hub and a flexbeam unit with flexbeam elements that are implemented as connecting members between each one of said rotor blades and said rotor hub. Each such connecting member comprises an intermediate elongated portion that is bent in the rotor hub, and end portions that are rigidly secured to the rotor hub and an associated rotor blade. Furthermore, each connecting member comprises a bundle of fibers, said fibers being agglomerated by a polymerized resin in said end portions and by a vulcanized elastomer in said intermediate portion. More specifically, each connecting member includes a composite longitudinal bundle of substantially unidirectional rods of high mechanical strength, in particular tensile strength, each of which possesses a certain degree of elasticity in flexion and is formed by the agglomeration of a large number of mineral or synthetic fibers and/or filaments by a polymerized synthetic impregnation resin, each rod being individually encapsulated by a flexible vulcanized elastomer having a relatively low shearing factor and considerable deformation remanence, and which forms a matrix linking together the rods to form the said composite longitudinal bundle.
However, an underlying total number of constituent longitudinal bundles of each flexbeam element is limited due to the fact that these bundles of fibers are bent on the flat in the rotor hub, hence along the direction where they exhibit the greatest bending stiffness, by an angle equal to that existing between two neighboring rotor blades. This entails the existence of substantially different lengths between the fibers of each constituent bundle and, thus, a limited distribution of mechanical stresses between the fibers of each such bundle.
The document U.S. Pat. No. 4,242,048 describes a semi-articulated eight-shaped flexstrap for use in connecting a rotor blade to a hub of a helicopter rotor. The eight-shaped flexstrap includes a series of straps which alternatingly overlap or crisscross at a point between the blade and hub. The straps are substantially parallel to the plane of rotation of the rotor at a given crossover point and unrestrained so as to permit relative movement between the straps. The relative movement of the straps of articulation of the eight-shaped flexstrap in the plane of rotation of the rotor permits lead lag motion of the rotor blade to occur.
More specifically, according to the document U.S. Pat. No. 4,242,048, the eight-shaped flexstrap comprises three continuous, uninterrupted bands, each defining two straps. Each band loops around a hub attachment fitting and a blade attachment fitting, thereby defining the eight-shaped flexstrap. All bands are secured to the hub attachment fitting and the blade attachment fitting by suitable bolts which respectively traverse all bands and threadably engage the corresponding fitting. In between the hub attachment fitting and the blade attachment fitting, each one of the bands respectively straps is rotated 90°, so that the edgewise face of each strap is flat or substantially parallel to the plane of rotation at a predetermined crossover point, when the two straps of each band cross over one another. In order to properly position and maintain this crossover point, a harness is provided, which secures the crossover point and which is itself fixed to the hub of the helicopter rotor.
Furthermore, multiple other connecting elements for connecting rotor blades to corresponding hubs of a helicopter rotor are known. By way of example, such connecting elements are described in the documents NL 88777, US 2012/087797 A1, GB 2 092 543 A and U.S. Pat. No. 5,286,170.